About 20% of amorphous silicon oxide (silica/SiO2) is contained in nonfood farm crops such as chaff or rice straw as well as carbohydrate such as cellulose or hemicellulose. Thus, conventionally, it has been tried that high-purity amorphous silica is extracted and used as a soluble silica fertilizer in order to recycle the nonfood farm crops. In addition, since such amorphous silicon oxide has high pozzolan activity, it forms stable calcium silicate by a hydration reaction with cement and as a result, the cement is densified and its strength is improved.
As a method of extracting silica from chaff, there is known a conventional method in which a blasting treatment, steaming treatment, supercritical water treatment or acid cleaning treatment using concentrated sulfuric acid or hydrochloric acid are performed on the chaff and the chaff after the above treatments is heated and burned. High-purity silica can be produced as combustion ash by the above methods. However, according to any one of the above production methods, the problem is that the production process is complicated and at the same time, productivity is low and an expensive high-grade material is used in a treatment container in view of corrosion resistance and environment resistance. In other words, the above production methods have an economic problem for a method of extracting combustion ash silica from chaff or rice straw in large quantity at low cost.
Some documents disclosing production methods of amorphous silica from chaff will be introduced below.
Japanese Examined Patent Publication No. 7-57684 discloses “production method of high-purity silica from chaff”. According to the method disclosed in this document, chaff is treated with a solution of hydrofluoric acid (HF) to extract an existing silicon compound contained in the chaff to produce silica. More specifically, the silicon compound after the treatment with the solution of hydrofluoric acid is extracted as hexafluorosilicic acid (H2SiF6) and the extract is added with ammonia and decomposed via ammonium hexafluorosilicate ((NH4)2SiF6) into ammonium fluoride (NH4F) and silica (SiO2), and this is separated by filtration, washed in water and dried to obtain amorphous silicon oxide powder. According to this method, the problem is that economic efficiency is lowered because expensive hydrofluoric acid is used and the material of an equipment to be used is expensive because hydrofluoric acid has strong corrosive aggressiveness.
Japanese Unexamined Patent Publication No. 2006-187674 discloses a “silicic acid-containing powder production method and silicic acid-containing powder production system”. According to the method disclosed in this document, chaff or straw is fed into a treatment container and steam is jet into the treatment container while an inner pressure and a temperature is increased to powder the chaff or straw without burning it. Thus, a straight fertilizer can be produced from the chaff or straw directly.
Japanese Unexamined Patent Publication No. 5-194007 discloses “production method of amorphous silica fine powder and concrete blended with amorphous silica fine powder”. According to the method disclosed in this document, amorphous silica fine powder is produced by immersing chaff in a solution of hydrochloric acid and burning and grinding it. When the chaff is cleaned with a hydrochloric acid aqueous solution having a concentration of 5%, SiO2 purity is about 96%, which is not satisfactory. In order to implement 98% or more of the SiO2 purity, it is necessary to prepare a hydrochloric acid aqueous solution having a concentration of 10% to 20%. In this case, since chlorine (Cl) remains in silica powder provided after burned at 600 to 700° C., when this silica powder is used as a concrete raw material, the chlorine could corrode reinforcing steel.
Japanese National Publication of PCT Application No. 2004-527445 discloses “production method of high-purity amorphous silica from biogenic material”. According to the method disclosed in this document, a long-chain hydrocarbon compound such as lignin or cellulose is reduced to further combustible (volatile) short-chain hydrocarbon compound by immersing a biogenic material such as chaff in a solution having an oxidizing solute (selected from a group comprising peroxide, nitrate salt and permanganate) to reduce an amount of carbonaceous residual material of silica produced by the next process of burning. The burning temperature at that time is preferably 500 to 950° C. Crystallization of silica is accelerated during the high-temperature burning process.
Japanese Unexamined Patent Publication No. 7-196312 discloses “production method of amorphous silica comprising chaff as raw material”. According to the method disclosed in this document, chaff is thrown into a fluidized bed and primarily burned in a short time at 640 to 800° C. for 4 to 10 seconds and then thrown into a rotary furnace of an external heating type and secondly burned at 700 to 950° C. for 8 to 15 minutes. According to this method, amorphous silica having a low content of unburned carbon and high pozzolan reactivity can be obtained.
Japanese Unexamined Patent Publication No. 8-48515 discloses “production method of amorphous silica from chaff as raw material”. According to the method disclosed in this document, by immersing chaff in water and then burning it, or by supplying or ejecting water or steam before or at the time of burning, high-purity amorphous silica is produced. An impurity adhered to a chaff surface is removed by immersing the chaff in the water or supplying or ejecting the water, and then the chaff is burned at 550 to 600° C.
S. Chandrasekhar et. al. published an article entitled “Effect of organic acid treatment on the properties of rice husk silica) in JOURNAL OF MATERIALS SCIENCE 40 (2005) 6535-6544. According to the method disclosed in this article, two kinds of organic aqueous solutions such as acetic acid and oxalic acid are prepared, the aqueous solutions having a concentration of about 0.5% to 30% are boiled and chaff is thrown into these aqueous solutions, stirred for 90 minutes, taken out of the aqueous solutions, washed in water, dried at 110° C. and burned at 700° C. for 2 hours, by which amorphous silica having purity of 93 to 96.7% is produced.
Japanese Unexamined Patent Publication No. 11-323752 discloses “production method of material with high content of silica, cellulose and lignin”. According to the method disclosed in this document, after blasting or steaming treatment of a woody material such as chaff, rice straw or bagasse (the woody material is immersed into a solution such as hydrochloric acid, sulfuric acid or nitric acid and heated and pressurized), the woody material is burned at 550 to 900° C. (more preferably, at 600 to 700° C.), whereby amorphous silicon oxide having SiO2 content of 95% or more is produced. In other words, to produce high-purity silicon oxide having purity of 96% or more, it is necessary to thermally dissolve and remove the impurity by heating and burning at a higher temperature. However, the problem is that such heating at the high temperature causes a crystal structure of the silicon oxide to become crystobalite (crystallized).
Japanese Unexamined Patent Publication No. 8-94056 discloses “incinerating method for organic waste”. According to the method disclosed in this document, organic waste such as chaff or straw is dry distilled or heated at 150 to 1000° C., and obtained carbide is cleaned and demineralized with an acid solution, an alkaline solution or a washing solution containing a metal chelate agent, and then the demineralized carbide is incinerated. The obtained demineralized carbide can be used as a cement admixture. The acid includes inorganic acid such as sulfuric acid, chlorine acid and nitric acid and organic acid such as formic acid and acetic acid. The metal chelate agent includes salt having metal complex forming ability such as ethylene diamine tetraacetic acid (EDTA), nitrilo toriacetic acid (NTA), and ethylene diamine tetrapropionic (EDTP). Thus, when the demineralizing process is performed before the incinerating process, a preferable cement admixture can be provided. In addition, since hydrogen chloride is hardly contained in incinerated exhaust fume, dioxine can be prevented from being generated.